1. Field of the Invention
The present invention relates to a noise reduction audio reproducing device and method whereby audio to be listened to can be reproduced comfortably even under a noise environment.
2. Description of the Related Art
Hitherto, a noise reduction technique has been proposed as a technique for realizing improvement in audio clarity by suppressing noise to perform audio emphasis. With the present Specification, noise reduction will be abbreviated as NR below.
For example, with a headphone system whereby users can perform conversation while putting on headphones, and with an earphone system having a hearing aid or hearing aid function (external monitor function), a system configuration can be conceived such as shown in FIG. 25A.
That is to say, though not illustrated in the drawing, for example, microphones 1L and 1R are attached to the outside of headphone casings (for the left ear and for the right ear) such as earmuffs with strong sound insulation and difficulty in attachment/detachment as an example of acousto-electric converter. Subsequently, after audio signals collected at the microphones 1L and 1R are amplified at a microphone amplifier 2, the audio signals are converted into digital audio signals at an A/D converter 3, and are supplied to an NR processing unit 4.
The NR processing unit 4 subjects the digital audio signals to NR processing to suppress noise, thereby performing audio emphasis. The digital audio signals subjected to audio emphasis are returned to analog audio signals at a D/A converter 5, and are supplied to a speaker or headphone driver unit through a power amplifier 6, and are reproduced acoustically.
The spectrum subtracting method (hereafter, abbreviated as “SS method”) described in “MATLAB multimedia signal processing, lower volume, audio, image, and communication” collective writing of Masaaki IKEHARA, Tetsuya SHIMATANI, and Yukitoshi SANADA, BAIFUKAN Co., Ltd issue, pp 67-74, for example, can be employed as the NR processing at the NR processing unit 4, and the system configuration in FIG. 25A can be rewritten such as shown in FIG. 25B. That is to say, the NR processing unit 4 is replaced with an SS-method processing unit 4A and musical noise removal filter 4B.
The SS method is a method for subtracting the power spectrum of the noise estimated separately from the power spectrum of an audio signal to which noise is added, subjecting the power spectrum thereof to inverse Fourier transform, thereby restoring an audio signal from which the noise is removed.
The power spectrum of the noise to be subtracted is estimated and stored in a storage unit beforehand. For example, with a soundless section of audio to be listened to, audio collected at the microphones 1L and 1R can be stored in the storage unit as estimated noise. If the power spectrum of the estimated noise is suitable, noise reduction effects are great. Subsequently, if the noise estimated as the power spectrum of the noise to be subtracted is steady noise, the noise is reduced by the SS method, and only the audio component to be listened to is emphasized. Though this SS method is a very simple algorithm, very high noise removal effects can be obtained.
Note that, in the case of employing the SS method, noise due to lack of phase information called as musical noise occurs, so it is desirable to employ a configuration wherein the musical noise removal filter 4B is provided on the subsequent stage of the SS-method processing unit 4A. Removal of this musical noise has been described in “Musical Noise Reduction Using Morphology Process in Spectral Subtraction” Hideaki TOZAWA, Yukihiro NOMURA, Noritaka YAMASHITA, Jianming LU, Hiroo SEKIYA, Takashi YAHAGI Graduate School of Science and Technology, Chiba University, The 18th Workshop on Circuits and Systems in Karuizawa, Apr. 25-26, 2005.